In August, Musk received approval from the City Council in Hawthorne, California, to build a two-mile test tunnel.

Musk jokingly referred to the fundraising via hat sales as an "initial hat offering," taking a dig at the initial coin offering seen in the cryptocurrency world. An ICO is when a company sells digital tokens in exchange for cryptocurrency, but investors don't get a stake in the firm. ICOs so far have got a bad rap, with one start-up called Confido raising $375,000 then disappearing with the money.

WATCH: Elon Musk shares first picture of a tunnel dug by his Boring Company